day 1 - liver and enzymes Flashcards
potential causes of Raised serum enzymes
Increased synthesis, including cell proliferation Tissue damage enzyme type indicates tissue source level indicates rate/amount of damage Cell death Defective clearance (“macro-enzymes”)
potential causes of Low enzyme activity
Artefactual - EDTA, citrate particularly important for inborn errors where gene defect may affect: - active site - Synthesis of enzyme Co-enzyme defect/deficiency
a potential problem with using the lab quoted reference range
Laboratory may not quote age/sex adjusted reference range on report.
Range probably for healthy adult
User must vary interpretation as required e.g. if a child, particular issue with ALP as this spikes during adolensence
Even if laboratory does quote age/sex range may still be other variation factors eg,
- ALP in later pregnancy (increases)
- CK in different ethnic groups (higher in Afro-Caribbean)
causes of raised creatine kinase
Myocardial infarction
- timing and practicalities of use (peaks at 24hrs and gone by 48 hrs)
- Superseded by Troponins
Rhabdomyolysis
- More sensitive than urine myoglobin
Hypothyroidism
Post exercise
Statin-induced myositis
Dystrophies (v. high CK)
use and problems of amylase
Isoenzymes: Salivary and pancreatic
Main use
- large rise in acute pancreatitis
- 10X ULN (upper limit of normal) in patient with acute abdomen virtually diagnostic
Problems
acute overuse (don’t need to measure daily)
Other causes of raised levels (e.g. mumps,CKD)
- macroamylase!
what is macroamylase
= Persistently elevated amylase with no clinical reason
- Check urine. If not raised probably due to macroamylase
=amylase + immunoglobulin, too big to be cleared by the kidney
N.B. Can get macro forms of many enzymes and proteins (macroCK, macroprolactin etc)
- If something doesn’t fit with the clinical picture, discuss with duty biochemist
major functions of the liver
Metabolism of: -Carbohydrate - Gluconeogenesis, glycogenolysis Lipid - Synthesis, metabolism Protein - Synthesis and catabolism Hormones - mainly inactivation drugs/toxins - activation/metabolism Immunological function Storage - e.g. glycogen
‘LFT’ request @UCLH includes
Bilirubin
Albumin
Alanine transaminase (ALT)
Alkaline phosphatase (ALP)
use of bilirubin as a test of liver excretory capacity
Bilirubin is one in vivo test of liver excretory capacity.
10-fold reserve capacity of liver makes it relatively insensitive.
Necessary to understand stages in bilirubin metabolism because things can go wrong at each stage.
different types of bilirubin
Serum bilirubin (3-17 umol/L)
Serum conjugated bilirubin 0-3 umol/L
Urine bilirubin - not normally detected
Urine urobilinogen
what is bilirubin
Bilirubin (formerly referred to as haematoidin) is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases. It is responsible for the yellow color of bruises, the background straw-yellow color of urine via its reduced breakdown product, urobilin (the more obvious but variable bright yellow color of urine is due to thiochrome, a breakdown product of thiamine), the brown color of feces (via its conversion to stercobilin), and the yellow discoloration in jaundice.
the production of unconjugated bilirubin
Erythrocytes (red blood cells) generated in the bone marrow are disposed of in the spleen when they get old or damaged. This releases hemoglobin, which is broken down to heme as the globin parts are turned into amino acids. The heme is then turned into unconjugated bilirubin in the monocyte macrophages system of the spleen. This unconjugated bilirubin is not soluble in water, due to intramolecular hydrogen bonding. It is then bound to albumin and sent to the liver.
processing of unconjugated bilirubin once it reaches the liver
In the liver, bilirubin is conjugated with glucuronic acid by the enzyme glucuronyltransferase, making it soluble in water: the conjugated version is also often called “direct” bilirubin.
Prematurity, Crigler-Najar syndrome (congenital enzyme deficiency), some patients with Gilbert’s syndrome (partial enzyme deficiency), inhibition by novobiocin or breast-milk steroids, can all interfere with bilirubin conjugation.
excretion of conjugated bilirubin from the liver
Excretion across the microvilli into the biliary canaliculi, where bilirubin is complexed with bile salts, phospholipids and cholesterol; and conveyed by the common bile duct into the small intestine.
Mechanical obstruction by cirrhosis, hepatitis, tumour, gallstone in the common bile duct, or carcinoma of the head of the pancreas may cause jaundice, as may interference by chlorpromazine, or the congenital Dubin-Johnson and Rotor syndromes.
processing of conjugated bilirubin in the intestines
In the large intestine, bilirubin is hydrolysed by bacterial enzymes to colourless urobilinogen ( which can be oxidized to form urobilin and stercobilin: these give stool its characteristic brown color.).
In patients taking antibiotics to sterilise the gut prior to surgery, there may be an absence of urobilinogen from the urine.
Urobilinogen is reabsorbed from the portal circulation by the liver, and re-excreted (enterohepatic circulation). Some enters the systemic circulation for excretion by kidneys.
In the pre-icteric stage of viral hepatitis, there can be a marked increase in urinary urobilinogen as the liver fails to take up urobilinogen from the portal circulation, leaving more to be excreted by the kidneys.
Classifying Liver Disorders
Pre-Hepatic
- Bilirubin predominantly unconjugated
Hepatic
- Bilirubin conjugated
Post-Hepatic
- Bilirubin conjugated
- Signs of impaired excretion e.g. pale stools
Pre-Hepatic liver disorders
Caused by increased bilirubin production: Ineffective erythropoiesis - Pernicious anaemia Excess rbc destruction - Immune anaemias, sickle cell anaemia
Bilirubin is unconjugated (insoluble) and therefore is not found in the urine
Hepatic liver disorders
Caused by liver cell damage:
Bilirubin is conjugated
- Soluble, and in urine (urine dark)
Excess urine urobilinogen
- Failure of liver to take it up after gut absorption and therefore more is excreted via kidneys
Raised aminotransferases (e.g. ALT)
Tests of Liver Function - Enzymes
Transaminases: aspartate transaminase (AST) alanine transaminase (ALT) Markers of hepatocellular damage but not liver specific Tend to use ALT
Transaminases are not specific for the liver.
AST widely distributed, especially in muscle including cardiac muscle and red cells.
ALT slightly more specific for liver disease but is also widely distributed
In damage to hepatocytes, intracellular contents leak into the circulation.
Cholestasis from whatever reason stimulates alkaline phosphatase synthesis and causes a rise in serum level. (Enzyme induction.)
ALP also found in bone, gut, placenta. Isoenzymes may be distinguished (with some difficulty).
GGT sensitive indicator of hepatobiliary disease, but is very susceptible to enzyme induction by drugs eg, anticonvulsants, and alcohol.
Whether disease is primarily hepatocellular or cholestatic is usually indicated by the relative rise of transaminase and ALP.
Cirrhosis LFTs
Biochemistry may be unremarkable depending on amount of functional tissue present.
Rare inherited causes of cirrhosis:
Wilson’s disease
Alpha 1 antitrypsin deficiency
what happens in Wilson’s disease
Liver and neurological presentations
Deficiency of a copper transport protein, ATP7A
Severe forms have copper deposition in the eye (Kayser-Fleischer rings)
Biochemical abnormalities:
Decreased serum caeruloplasmin
Increased output of urinary copper
what happens in Alpha 1 antitrypsin deficiency
Neutrophil elastase inhibitor
Acute phase reactant
Measure with specific assay
Inherited deficiency states, e.g. PI ZZ, cause cirrhosis and emphysema
Included in differential dx of prolonged, conjugated jaundice of neonate.
Post-Hepatic liver disease
Many causes: Gallstone Tumour (primary or secondary) Biliary atresia Not distinguishable biochemically
Bilirubin is conjugated (in urine)
Urine urobilinogen absent
Raised ALP (induced synthesis) and GGT
sources of ALP and problems
Liver and Bone sources Liver: Obstruction > 3xULN Bone: Osteoblasts = growth/turnover Isoenzyme analysis possible Problems: Gut and placental sources of ALP may cause confusion Sample contamination, EDTA Reference ranges change with age
what is GGT
Serum gamma glutamyl transferase (GGT) enzyme induction and cholestasis Induced by many drugs, including ethanol Raised in all hepatic and post-hepatic jaundice i.e., not discriminatory Raised GGT does NOT mean alcoholism
Tests of Liver Function - Proteins
Albumin
- Maintenance of conc. can be achieved with only 10% of hepatocyte mass
Prothrombin and other clotting factors
- INR (international normalized ratio)
Serum immunoglobulins G,A,M
- Not synthesised by liver but secondary changes common
what are macro enzymes
This refers to the binding of an enzyme to immunoglobulin, thus decreasing its rate of degradation.
Macro enzymes may occur where there is a primary protein (immunoglobulin) disease, such as myeloma, but may occur unexpectedly.
A macro-enzyme in a patient may be temporary.
CK and AMY commonly occur as macro enzymes.
If present, may need to use alternative methods for assessing tissue damage (e.g. if
macro CK-MB present, use troponin) and need to document to avoid re- investigation of patient.
what are troponins
Myofibrillar proteins (rather than an enzyme) released from damaged cells.
Several types, Troponin-T and Troponin-I cardiac specific.
Rise of level after event similar to CK and CK-MB but high sensitivity assays can be used much earlier (on admission) with a repeat analysis at 3 hours if not initially above the 99th centile. Elevation is more persistent, up to 10 days, therefore can be useful in late presentations.
Considered also to demonstrate low-grade myocardial damage short of infarction: possibly pre-infarction states requiring acute treatment.
Good rule-out properties when used in appropriate time frame.
what does ALT stand for
- Alanine transaminase (ALT). A cytosolic enzyme
what does AST stand for
- Aspartate transaminase (AST). Current use with ALT in identifying non-alcoholic
steatohepatitis (NASH) in primary care (AST/ALT ratio)
what does ALP stand for
- Alkaline phosphatase (ALP). A biliary brush border enzyme
what does GGT stand for
- Gamma glutamyl transpeptidase (GGT). A second line test reflecting ribosomal activity.
The use of these enzymes will be mainly discussed below in the section on liver function. Note that GGT is NOT in any way specific for alcoholic liver disease. This is a common misconception. The enzyme can be induced by a number of factors.
what is icterus
Jaundice (icterus) is the clinical manifestation of raised plasma bilirubin and is a
yellow discoloration of the skin or sclera visible at bilirubin levels above 50
micromol/L (3xULN).
A raised unconjugated bilirubin suggests
A raised unconjugated bilirubin suggests haemolysis or failure of conjugation.
A raised conjugated bilirubin suggests
A raised conjugated bilirubin suggests defective excretion.
Bilirubin detected in the urine must be conjugated.
Alanine transaminase (ALT) and aspartate transaminase (AST) are released from
Alanine transaminase (ALT) and aspartate transaminase (AST) are released from damaged liver parenchyma cells. Rises in serum levels are found in many non-specific liver diseases. The highest levels are found in viral hepatitis or hepatic necrosis.
Alkaline phosphatase (ALP) is synthesised by
Alkaline phosphatase (ALP) is synthesised by the liver and the osteoblasts.
Raised levels are found in obstruction especially primary biliary cirrhosis, because synthesis is induced.
Raised levels are also found in metabolic bone disease and may often be distinguished by measuring GGT which usually rises and falls with liver ALP.
Alternatively, one can use assays which specifically measure the bone and liver isoenzymes
Serum gamma glutamyl transferase (GGT) is raised in
Serum gamma glutamyl transferase (GGT) is raised in drug-induced enzyme induction and cholestasis including, but not specifically, alcoholic liver disease.
